Testing the Canon 40D & 17-40L in the S&S NX Dome

Interests:Protecting our Ocean, Environmental Education,
Having fun and Living Well

Posted 09 June 2009 - 10:39 PM

In preparing for a JASA wild dolphin trip next week I've been trying to determine the best port configuration for my 40D using the 17-40L. The Sea & Sea chart recommends the NX Fisheye dome with no extension. My early simple checks in the pool showed some very soft corners with this setup. In talking to four respected shops in the US, I got multiple and different recommendations that ran the gamut of possible configurations.(No extension, the 20 extension, the 40 extension with a +2 diopter) At first blush I couldn't understand how this was possible but after a few hours of tests in the pool I'm starting to get the picture. Based on my initial tests the "best" configuration is different depending on the image you're making, the aperture, and whether sharp corners or lack of distortion is most important to you. The short answer I found is to use the 40 extension ring, shoot between f8 and f11 and use a +2 diopter if your image DOF allows it.

Here is the test setup I used to try and figure this out. I printed up some 13x 19 lens focus charts from http://www.graphics..../res-chart.html, laminated them and taped them to a 4' x 6' pvc frame placed in my pool.

I shot a sequence of apertures; 4, 5.6, 8, 11, 16, 22 at 17mm, 29mm, and 40mm for each of four configurations with my NX fisheye dome.

I shot at ISO 100 with AF on, targeted at the center point of the center lens chart. Exposures were adjusted manually and the testing took place in the bright Arizona sunshine. My strobes were used at fractional powers to provide some fill for the lower half of the pool. After the run the images were loaded into Lightroom and keyworded with the configuration notes. LR is a brilliant tool to use for this kind of work. I was able to easily filter for an f-stop then use the comparison screen to put two 1:1 and 2:1 crops side by side to decide which appeared to have the best focus/sharpness.

On my initial runs through the images I was primarily looking for sharp corners. Some of the choices are obviousand others required some pixel peeping to decide. All of this is pretty subjective on my part. You might make a different choice in some cases. The images I've included are 1:1 crops of the test chart in the bottom left corner. Might be a 2:1 in here somewhere. Here is what I think I found;

-The center is sharp in all cases I tested-f/8 and f/11 is the sweet spot with markedly better corners than even f/5.6

f/4 is not worth shooting if you can help it.

f/8 to f/11 is a touch better than f/16-f/22, which I didn't expect. 2:1 crop

The Sea & Sea recommended configuration provided the worst corners in all cases. The 40 Ext on left, no ext on right.

Interests:Protecting our Ocean, Environmental Education,
Having fun and Living Well

Posted 09 June 2009 - 10:46 PM

The 40 Extension is obviously sharper in the corners than the 20 for all apertures (40 on left)

I also did the same set of comparisons zoomed to 40mm by tightening up on just the center focus chart,

And got the same results as when shooting the whole test frame. This is the 40 extension with diopter versus no extension;

So what does all this mean when I finally have a chance to get in the water? I'm thinking for regular shots of a six foot dolphin type subject shot 2m away the ext 40 is the clear choice. If I think I can get closer it makes sense to have the diopter on. The main difference in the image will be the blurred background when using the close up lens. I'm still trying to get my head around how that will look in open water. If any of you can give me any guidance I'd love to hear it. Aperture is so important with this lens setup that I'm thinking I'll shoot Av and bump the ISO to get the exposure I need to stop the fast moving dolphins. At least now I'm comfortable that I know what to expect from the lens / port combination.

I'm very new to this type of testing, so if I've missed something or am just confused please let me know.

Excellent work there Steve. A very impressive contraption that you built!

Interestingly, in the Subal line ( and it appears that the port/lens recommendations are generally very similar ) the suggested combination for the 17-40mm lens was initially a 40mm extension ring and a +2 diopter. That is what I have and although I dont use it that often as I have become a bit cornersoft phobic, it works OK. However their recommendation was updated( http://www.subal.com...;markennummer=1 ) to a 50mm extension ring with +2 diopter. I have been meaning to try that out but dont currently have that extension. Just a thought - maybe you will get even better results with a 50mm extension ring???

Thanks for posting - and if you could test a 50mm extension I will be very interested.

Interests:Protecting our Ocean, Environmental Education,
Having fun and Living Well

Posted 10 June 2009 - 04:29 PM

Thanks guys, you mean all I had to do was look at the Subal charts? Damn, now you tell me I wonder what the Aquatica and Seacam folks think?

Andre, I don't think S&S makes a 50 so I'm out of luck there. Thanks for the comment on the "contraption", around here it's unaffectionately known as the "rack". My wife knows better than to leave me alone for a weekend with excess building material.

Phil, I'm taking my 10-22 along, It's nice that the 77mm dioptre fits both lenses. I'll see if I can give it a try.

I have only just stumbled across this Steve. Looks like you certainly done your homework. So from what I can make out ( affected by drink at the minute ) the 40mm extension is good for general wide angle but for CFWA you would go with the +2 aswell. I think Mrs Williams should leave you alone for more weekends to experiment.

Interests:Protecting our Ocean, Environmental Education,
Having fun and Living Well

Posted 07 August 2009 - 03:49 PM

(Sorry Mr. Williams. I just had to. First round on me next time we meet up.)

Thanks Stew,I think you have it right as long as you stay around f/11,

Drew, I may have to go a day early to San Diego this year just so you can buy me all the rounds we have stacking up. By the way I think you may be on to something about my lady's motives, but I'm not complaining.

Thanks guys, you mean all I had to do was look at the Subal charts? Damn, now you tell me I wonder what the Aquatica and Seacam folks think?

Cheers,Steve

Hi Steve - Nicely controlled test with sound deductions. In answer to your question, we recommend a PVL 35 with the Seacam superdome for both the old style 16-35 and the 17-40. For the newer (and longer) 16-35II I use a 57.5mm port extension. That's a 9" dome, so there might be variables in directly interpolating our data to the Sea and Sea dome, but interesting that the results are so similar.

BTW ... Unless there is a significant optical advantage to a 50mm extension over a 40mm, I wouldn't do it. The longer the port extension, with most housings, the greater the upward torque from the air in the dome + extension, so at some point there are diminishing ergonomic rewards. As you've observed, shooting at F8, and preferably F-11, will cure a lot of corner angst.

I just want to add one little wrench into the works here, that i think we all have to keep in mind, that is FX vs DX. From the comparisons I've done with my D200 and D700, with a rectilinear lens, a full frame set-up will lose you one stop or so in terms of keeping equivalent sharpness. I do not find this to be the case with fisheye - another reason I am moving more and more to fisheye with TC rather than wide rectilinear. This is my experience with the Nikon 12-24, 17-35, 16 and Tokina 10-17
I think with full frame a diopter also becomes more important with full frame to maintain corner sharpness. Steve, I applaud your little rig, if it's OK with you I'm going to copy it. It will be great if we can all get things standardized like this with sizes, distances etc.

Interests:Protecting our Ocean, Environmental Education,
Having fun and Living Well

Posted 08 August 2009 - 09:15 AM

No problem Jeff, copy away. One thing you might think about. It's possible to put a Tee in the rig so that a piece of PVC pipe can come out vertically from the plane of exposure. (You can see it in the first pic above) If you do that you can use a ULCS clamp to hold your camera rig in a fixed position and be a known measured distance from the frame. Just one more variable to take out of the equation. You can use your focus light adapter on the top of the housing to clamp to the pipe. If you put the tee in the right place on the frame the lens will always be centered on the test image. One other point, might not want to glue all the joints. If you use a press fit on some of the horizontal pieces (each corner) you can take it apart and stow it a little easier. Have fun!

Stephen, glad you could corroborate my little test. I'm with you completely on the balance of the rig. The more I shoot, the more the appreciate the importance.

I cant wait to use my 17-40L underwater. I had it on my MK2 yesterday ( topside ) and really like it. The corners are so much better than the 10-22 use to give me. For my UW set up this test is perfect. I wish I had a pool where I could do tests like this and perhaps give a bit of information back to the forum. When I move to Egypt I will be able to do it everyday. Once again a BIG thank you to Mrs Williams.
It certainly deserves to be on the WP home page as I know this test will be looked at by a lot of guys and girls.

No problem Jeff, copy away. One thing you might think about. It's possible to put a Tee in the rig so that a piece of PVC pipe can come out vertically from the plane of exposure. (You can see it in the first pic above) If you do that you can use a ULCS clamp to hold your camera rig in a fixed position and be a known measured distance from the frame. Just one more variable to take out of the equation.

One thought on this; as you zoom in or out, would you agree that the distance needs to varied, so that the frame fills the image frame for any given focal length?

Interests:Protecting our Ocean, Environmental Education,
Having fun and Living Well

Posted 09 August 2009 - 05:53 AM

One thought on this; as you zoom in or out, would you agree that the distance needs to varied, so that the frame fills the image frame for any given focal length?

Yes, completely agree, if you decide to use the alingment pipe coming out of the frame you can mark it with a sharpie for the proper distances for each zoom setting/lens. That way you can do very repeatable tests for each sequence. It takes a fair amount of time to do these tests, working your way through all the f stops and zoom settings. (by the way it helped to have a little checklist) I found it was much faster not to use the clamp and just use my eye but as you suggest the framing is critical if your working on understanding the absolute best corner sharpness. You can think of it as a monopod attached to the test frame.